The present invention relates to prosthetic knee joints and more particularly to a device for and method of increasing the effective thickness of a femoral knee joint component.
Knee arthroplasty is becoming more common to partially or totally replace knee joints which have been damaged due to trauma or disease. In some cases, only one component of the knee joint, the tibial component or the femoral component, needs to be replaced, while, in other cases, both components of the knee joint need to be replaced. Knee arthroplasty requires that the end of the bone of the patient be cut and shaped to receive each component of the prosthesis in proper alignment and, thereafter, insuring that each component is properly sized to permit the repaired joint to function as normally as possible.
In some instances, the bone is so badly diseased or altered due to a previous knee implant that it requires that the bone be cut significantly in order to receive the prosthetic component. When this occurs and a typical knee joint is implanted, the leg will be shortened. One way surgeons avoid this problem is by implanting a thicker knee component to compensate for the lost bone. For example, some femoral component designs are available in various thicknesses for various sizes. For example, Howmedica, Inc. sells the P.C.A. Revision Total Knee System, which, as shown in Form #H2030 3/83 15M B (1983), has small, medium, and large femoral components, and the medium and large sizes are each available in two thicknesses. However, this solution requires the manufacturer to make and the surgeon to stock several joint components, to cover the various thicknesses and sizes to satisfy the patients' needs. This is a costly and cumbersome procedure for the surgeon and the manufacturer.
Another way for the surgeon to increase the effective joint thickness during knee arthroplasty is by using an implantable tibial base with removable tibial base insert. The tibial base insert may be of various thicknesses and be made to fit several sizes of tibial bases. The P.C.A. Revision Total Knee System is an example of such tibial components.
However, for either solution, using a thicker femoral component or a thicker tibial insert, an oversized component to compensate for loss bone could result in poor ligament balance, improper positioning of the joint line, and shifting of the patella position which could cause poor tracking of the patella on the femoral component. In addition, the surgeons that choose to use such knee prostheses are limited to the sizes supplied by the manufacturer and have no means of increasing the effective thickness beyond the thickest femoral component or tibial insert.
A third method some surgeons use to increase the effective length of the bone is to place a piece of bone between the cut end of the long bone and the knee component. This method has the disadvantages that the surgeons must acquire the bone material and carve the piece, and it most likely results in poor fit and fixation of the joint component.
It is known in the prosthetic art to incorporate cement spacers for controlling the thickness of cement applied between a prosthetic insert and a support member, such as an acetabulum. An example of such spacers is taught in U.S. Pat No. 4,417,571 to Nelson, et al. which discloses a prosthetic cement spacer and a method of using such spacers. One embodiment comprises a standoff body having a top surface and base surface and a means for anchoring the standoff body to the supporting member where the anchoring means extends outwardly from the base surface, e.g. a wire. The standoff body is preferably substantially cylindrical with a flat top surface, and the spacer is small enough to provide point support rather than a larger area support in order to be suitable for a variety of supporting bone shapes.
The method disclosed in Nelson, et al. comprises selecting a plurality (preferably at least three) of such spacers of desired height, positioning each against the supporting member by fully inserting the wires into the supporting member, filling the area between the supporting member and the prosthetic insert with cement, and placing the prosthetic insert against the outmost ends of the standoff bodies. This method is cumbersome and time consuming in that the surgeon must select several spacers, determine their placement, and attach each to the bone, one by one. Additionally, these spacers require that the surgeon take the time to apply and sculpture cement to fill in the space between the supporting member and the prosthesis.
Another example of spacers for controlling the thickness of cement are those shown in U.S. Pat. No. 4,563,778 to Roche, et al. which discloses a prosthetic acetabular cup of integral construction including a polyethylene liner in intimate contact with a metal shell and a plurality of standoff or spacer devices, preferably round or hemispherical in shape, that are securely located on the liner. Since the spacers are not removable, the surgeon has no flexibility in varying the height of the spacers, which are preferably sized to an overall height of 3 mm beyond the outer surface of the acetabulum cup to achieve optimum cement thickness.
The surgical procedure for using the spacers of Nelson, et al. and for inserting the acetabular cup assembly of Roche, et al. is to first prepare the acetabulum by reaming the bone surface to an increased spherical diameter to accommodate the additional height of the spacers. Therefore, the spacers and method disclosed do not teach a way of increasing the effective thickness of a knee joint component.
U.S. Pat. No. 3,683,421 to Martinie discloses a prosthetic hip joint assembly which is seated in the acetabulum opening. Martinie describes that the opening may be reamed out and provided with a plurality of grooves or cutouts 54 to more firmly hold the layer of acrylic 56 within which the socket assembly is mounted. Acrylic 56 appears to be cement that is typically applied just prior to inserting an socket assembly.
In the art there are known ways to patch proximal tibial defects when implanting the tibial component of a knee prosthesis; these ways are described in a paper entitled "Tibial Component Fixation in the Presence of Deficient Bone Stock" written by P. Brooks, P. S. Walker and R. Scott of Brigham & Women's Hospital in Boston, MA, and the V.A. Medical Center of West Roxbury. The article, published for the Feb. 7-9, 1984, 30th Annual ORS in Atlanta, Ga., shows that standard tibial trays can be supported on a defective tibia by (1) cement only, (2) cement, reinforced with two cancellous screws, (3) a Plexiglass (sic)--PLEXIGLAS.RTM., a trademark of the Rohm & Haas Co,--acrylic plastic spacer cemented into the defect, (4) a stainless steel spacer cemented into the defect, and (5) a custom made tray designed to match the defect. This teaching describes ways to patch bone defects but does not show a means for increasing the effective thickness of a knee joint component.
In DePuy's 1979 product catalog, an asymmetric tibial plateau formed of UHMWPE is shown as part of the Townley Anatomic Total Knee Replacement. This tibial plateau is asymmetric on its distal side shaped to fill in a portion where the proximal tibia is lacking. It is believed that this tibial plateau is attached to the cut end of the tibia by cementing it to the surface. This product provides a means of filling in a bone defect but does not provide a means for increasing the effective thickness of a knee joint component.